tral point of no solid substance is the actual spot in which its attractive power is lodged, yet it has been abundantly proved by Sir Isaac, that all the matter of a spherical body may, in generally estimating its attractive force on other matter, be considered as collected in the centre of such sphere. And hence, as all the celestial bodies are nearly spherical, their action on bodies at a distance may be regarded the same as if the whole of the matter of which they consist were condensed into their respective centres. To what extent in the heavens the power of gravitation ranges it is impossible to determine; there can be little doubt, however, that it extends from one fixed star to another, although its effects have not yet been calculated. It may possibly influence the progressive motion of several of the stars, and, as I had occasion to observe in a preceding lecture, is the cause to which Dr. Herschel ascribes the origin of the material universe, which he supposed at one time, though he seems afterwards to have modified his opinion, to have issued from an immense central mass of matter, peculiarly volcanic in its structure, and to have been, consequently, thrown forth in different quantities, and at different times, by enormous explosions; each distinct mass, thus forcibly propelled, assuming, from the common law of projectiles, an orbicular path, and endowed with the common property of the parent body, ejecting in like manner minuter masses at different periods of time, which have equally assumed the same orbicular motion, and ultimately become planets to the body from which they have immediately issued, and which constitutes their central sun. To produce such an effect, however, whether according to this theory or to others more generally admitted, and in reality to produce any of the motions which occur to us in the celestial bodies, the PASSIVITY of matter is just as necessary as its gravitation. I have already observed that, owing to its passivity, or VIS INERTIÆ, matter has a tendency to persevere in any given state, whether of motion or of rest, till opposed by some exterior power; and that the path it assumes must necessarily be that of a right line, unless the power it encounters shall deflect it into a different direction. A projectile, therefore, as a planet, for example, thrown forth from a volcano, would travel in a right line for ever, and with the exact velocity with which it was thrown forth at first, if there were nothing to impede its progress, or to alter the course at first given to it. But the attraction of the volcanic sphere from which it has been lanched does impede it, and equally so from every point of its surface: the consequence of which must necessarily be, that every step it advances over the parent orb it must be equally drawn back or reined in, and hence its rectilinear path must be converted into the curve of a conic section, and a tendency be given to it to escape in this line, which may be contemplated as a line of perpetual angles, instead of in a direct course; and as soon as the projectile or planet has acquired the exact point in which the two antagonist powers precisely balance each other the power of flying off from the centre, communicated to it by the volcanic impulsion, and which is denominated its CENTRIFUGAL FORCE, and the power of falling forward to the centre, communicated by the attractive influence of the aggregate mass of matter, which the parent sphere contains in itself, and which is called its CENTRIPETAL FORCE it will have assumed its proper orbit; and, through the influence of this constant antagonism of the two properties of passivity and gravitation, of a centrifugal and centripetal force, persevere in the same to the end of time. This, however, must be regarded as an illustrative speculation. Of the immediate cause of gravitation, or the nature of that power which impels different bodies to an union, we are in a very considerable degree of ignorance; or rather, perhaps, may be said to know nothing at all. It is necessary, however, to notice one very singular phænomenon concerning it, and to give a glance at two out of various theories by which gravitation has been attempted to be accounted for. The phænomenon is, that although, owing to this power, all bodies have a tendency to come into contact, they never come into actual contact; some kind of pore or open space being still left between the corpuscles of bodies that approach the nearest to each other. Thus, a plate of heated iron, solid as it appears to be, and altogether destitute of pores, becomes contracted in every direction by cold. So, too, as I have already observed, equal measures of water and alcohol, or of water and sulphuric acid, have their bulk sensibly diminished. In like manner Newton has remarked, that when two plates of glass are within about a ten thousandth part of an inch of each other, using fine metallic plates as a micrometer on this occasion, they support each other's weight as powerfully as if they were in actual contact, and that some additional force is requisite in order to make them approach still nearer. Nor is the force necessary to produce this effect of trivial moment: Professor Robison has calculated it, and has ascertained by experiment that it is equal to a pressure of about a thousand pounds for every square inch of glass. Air is not necessary to this resistance, for it is equally manifest in a vacuum ; yet it is a very curious fact, that under water it almost entirely disappears. It is, however, highly probable that the contact is never perfect, otherwise the two plates might be expected to cohere in such a manner as to become an individual mass. It is hence highly probable, if not absolutely certain, that matter, from some cause or other, is possessed of a REPULSIVE as well as of an ATTRACTIVE force; and that, like the latter, although its law has not been hitherto exactly ascertained, it increases in a specific proportion to its decrease of distance, or, in other words, as bodies approximate each other. It has hence been said, and this is the common theory of those who regard gravitation as an essential property of matter, that matter is universally endowed with two opposite powers; by the one of which material substances attract each other, and induce a perfect union; and by the other of which they repel each other when they are on the point of union, and prevent a perfect contact. It is admitted, however, on all hands, and is indeed perfectly clear in itself, that the repulsive power is of an almost infinitely less range than the attractive. It is believed that the attractive power, or that of gravitation, operates from world to world; yet the repulsive power can never be exerted, except "between such particles as are actually, or very nearly, in contact with each other; since it requires no greater pressure, when acting on a given surface, to retain a gallon of air in the space of half a gallon, than to retain a pint in the space of half a pint, which could not possibly be, if the particles exercised a mutual repulsion at all possible distances." * This idea, however, of double and opposite powers co-existing in the same substance, and in every corpuscle of the same substance, has been uniformly felt difficult of admission by the best and gravest philosophers; and hence Sir Isaac Newton, while allowing the repulsive power of matter, which in truth is far more obvious to our senses in consequence of its very limited range, has felt a strong propensity to question gravity as forming an essential property of matter itself, and to account for it from another source. "To show," says he, “that I do not take gravity for an essential property of bodies, I have added one question concerning its cause, choosing to propose it by way of question, because I am not yet satisfied about it, for want of experiments." In this question he suggests the existence of an ethereal and elastic medium pervading all space; and supports his supposition by strong arguments, and consequently with much apparent confidence, deduced from the mediums, or gases, as they are now called, of light and heat, and magnetism, respecting all which, from their extreme subtilty, we can only reason concerning their properties. This elastic medium he conceives to be * Dr. Young's Lect. vol. i. p. 612. |